Various devices are known which have multiple workstations and a mechanism for lifting and carrying workpieces from one station to another. In one device, a shuttle underlies the workpieces and is raised and lowered generally vertically by elevators or lift mechanisms underlying the shuttle. The elevators or lift mechanisms are actuated by a drive mechanism to raise the shuttle and hence lift the workpieces from their current workstations. The shuttle is advanced to transfer each workpiece to a successive workstation and then lowered to deposit each workpiece in its successive workstation and subsequently the shuttle is retracted to its starting position.
With the elevators or lift mechanisms underlying the shuttle, the shuttle has a beginning or lowest height which is raised from the floor. To ensure that the shuttle can clear and be moved below the workpieces when received in their workstations, the workstations must have a sufficient height above the floor and above the shuttle in its lowest position. Further, to ensure that the workpieces clear their workstations when lifted by the shuttle, a relatively long vertical stroke of the shuttle is required. Each of these factors results in undesirably excessive vertical height above the floor of the workpieces when deposited on their workstations, and instability of the shuttle and workpieces when raised. Moreover, the elevators or lift mechanisms and any drive mechanisms underlying the shuttle are highly susceptible to becoming contaminated and malfunctioning due to dirt, chips and the like falling off the workpieces and fouling the mechanism.
Another type of device which is used for transferring workpieces to consecutive workstations is commonly referred to as an overhead lift and carry device. A typical overhead lift and carry device has its lifting mechanism and shuttle disposed above the workstations and workpieces thereon. A plurality of clamping or gripping devices are carried by the shuttle and are separately actuated to grip each workpiece from above the workpiece to lift it off of a workstation so that it may be carried to a successive workstation. The overhead lift and carry device solves some of the problems of the lift and carry devices wherein the lifting mechanisms and shuttle are disposed below the workpiece but has an undesirably long cycle time, increased complexity due to the addition of the clamping devices and may be less safe because the parts are supported and carried from above and thus may be dropped should a clamping device fail. Further, repair hoists or the like cannot be readily moved above the workpieces because they will interfere with the overhead shuttle and clamping devices.
To complete one cycle of a typical overhead lift and carry device, the shuttle must be lowered to dispose the clamps adjacent to their respective workpieces on the workstations. The clamps must then be closed on each workpiece to grip it and the shuttle must be raised vertically to remove the workpieces from the workstations. Thereafter, the shuttle is advanced to dispose the workpieces above successive workstations and then lowered into these successive workstations. The clamps must then be released or opened to deposit the workpieces in the workstations and the shuttle raised and thereafter returned to its original position to begin another cycle. Notably, this eight step cycle takes a considerable amount of time and thus, lowers the rate of production of the workpieces and thereby increases the cost to manufacture the parts. Further, the increased complexity of the overhead lift and carry device along with the increased number of moving parts of the device decrease its efficiency and reliability in use.